nspawn,shared: cleanup use of ERRNO_IS_SECCOMP_FATAL()

nspawn,shared: cleanup use of ERRNO_IS_SECCOMP_FATAL()

Given that ERRNO_IS_SECCOMP_FATAL() also matches positive values,
make sure this macro is not called with arguments that do not have
errno semantics.

In this case the arguments passed to ERRNO_IS_SECCOMP_FATAL() are the
values returned by external libseccomp function seccomp_load() which is
not expected to return any positive values, but let's be consistent
anyway and move ERRNO_IS_SECCOMP_FATAL() invocations to the branches
where the return values are known to be negative.